Planetary Nebula what s left behind of the stellar core? Quick Recap. Supernova of a massive star what s left behind of the stellar core?

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1 Planetary Nebula what s left behind of the stellar core? Quick Recap Supernova of a massive star what s left behind of the stellar core?

2 Compact Stars and Degeneracy Pressure Recall that pressure from electrons can support white dwarf stars from collapse as longs as they contain less than 1.4 solar masses worth of matter. Neutron stars are supported against collapse by pressure from neutrons as long as they contain less than 3 solar masses worth of matter. gravity pressure Schematic of a star s entire existence! It s tough weighing over 1,000,000,000,000,000,000,000,000,000,000 pounds!!

3 Supernovae of Very Massive Stars After a massive star supernova, if the core has a mass > 3 times the mass of the sun, the force of gravity will be too strong for either the pressure from electrons or neutrons to resist. What happens if the core can no longer resist the force of its own weight? The core collapses in on itself completely and utterly. GRAVITY FINALLY WINS!! gravity

4 Black Holes The core of the star that went supernova becomes infinitely small, with a correspondingly infinite increase in the strength of gravity near the infinitely small point! Yikes!! This is what we call a black hole. Because it s gravitational strength is enough to prevent even light from escaping, the object appears dark it emits and reflects no light.

5 But wait light has no mass, so how can gravity affect light? According to Einstein s Theory of Relativity, gravity is really a distortion of spacetime, caused by objects with mass. Because light like everything else in our universe moves through spacetime, this means that light is affected by gravity, even though photons themselves have no mass. heavy

6 This means, for example, that any large mass can bend light and affect what you see when you look near it, just like a lens does in this case a "gravitational" lens! Note that the lens doesn t have to be a black hole any large mass will do the trick!

7 Gravitational Deflection by the Sun The amount of lensing caused by the mass of the Sun was, for instance, an early test of Einstein s theories on spacetime. Experimental verification of his predictions during the total solar eclipse of 1919 made Einstein quite famous. From the London Daily News, Nov. 1919

8 Warping of Spacetime by Gravity The apparent bending of light happens because gravity warps spacetime in a manner much like the way that people standing on a trampoline distort the surface of the trampoline. Each person makes a pit on the rubber sheet, warping it and affecting the motion of other objects on the trampoline.

9 So any object s path through such a warped spacetime will be bent, and under the right conditions can be made to orbit in a closed loop. As photons of light move through spacetime, they are also bent, and can also be caught in stable orbits when they come too near a black hole!

10 The sphere of points surrounding a black hole where light actually orbits the black hole is called the Event Horizon. Within the radius of this sphere, light cannot escape the infinitely steep walls of the gravitational pit in spacetime, and eventually trapped photons spiral in to the singularity at the center. This means that no light can ever escape, and no events inside the event horizon can ever be observed by the outside universe!

11 However, these mis-dimensional analogies do lead to misconceptions

12 A better visualization of an isolated black hole would look like the image on the right. Notice how the black hole is gravitationally lensing the light of stars behind. You d be more likely to see a black hole in a binary system, however, where it can interact with gas stripped away from its companion star to create swirling disks and streams of high-energy radiation.

13 More Misconceptions Do Black Holes Suck? At a distance, a black hole exerts gravitational force according to Newton s Law, just like a normal star with the same mass. So there s nothing unusual about a black hole s gravitational attraction unless you get very close. For example, if our Sun was replaced by a 1 solar mass black hole, the orbits of the planets would not change! You must be within 3 times the size of the event horizon from the black hole before gravity increases from what Newton s Law predicts (that s within 6 miles, for a 1 solar mass black hole!). Only then would the warping of spacetime cause an otherwise stable orbit to spiral into the black hole!

14 Gravity and Spacetime When you re that close to a black hole, though, the warping of spacetime affects many things besides the spatial orbit you re forced to follow. Time slows down in strong gravitational fields, an effect that s small but measurable even here on Earth. However, near much denser objects like white dwarfs, neutron stars, and black holes, this occurs to a much more significant degree.

15 Time Near a Black Hole If we launched a probe into a black hole, time as measured by an outside observer would appear to actually slow down for the probe as it approached the event horizon. For example, at some distance it would take 50 min of time on the mother ship for 15 min to elapse on the probe.

16 Also, light leaving the probe finds it harder and harder to get back to the mother ship: Redshifting of Light by Gravity Light from the probe is redshifted as it loses energy in trying to escape. The probe would eventually disappear from view as light from it is red-shifted beyond radio. Note however that light from the moment the probe crosses the event horizon can never get out to be seen by the mother ship at all!

17 And from the probe s point of view: Time for the mother ship appears to be sped up! The probe heads straight into the black hole without delay. Light coming from the mother ship appears blue-shifted to higher and higher frequencies!

18 But really, Tidal Forces destroy the probe well before it reaches the Event Horizon! As matter comes into such strong gravitational fields, the difference between the gravitational force felt by our astronaut s feet is significantly stronger than the gravitational force felt by our astronaut s head! Any physical object would be spaghettified!

19 Finding Black Holes Not Easy!! While we can t see radiation coming directly from a black hole, we can see the effect that a black hole has on its environment such as when a black hole has a stellar companion. In the 1960 s, such exotic pairings were suggested as a possible solution for the mysterious X-ray binaries. In these systems, a massive star is orbited by a smaller object with an accretion disk that emits unusual amounts of high-energy radiation. The X-ray binary Cygnus X-1 was the first good candidate for a black hole

20 Finding Black Holes in X-ray Binaries

21 Observations of the visible star s motion along with Kepler s 3rd Law gives a mass > 3 solar masses for the unseen companion in the Cygnus X-1 system. Such an object is too massive to be a neutron star or white dwarf The only thing that can be so massive, yet small enough to be invisible is a black hole.

22 Many other black holes have been found by measurements of their gravitational lensing effects. However, the majority have been found by analysis of the orbits and the energies of objects in the immediate environment of the suspected black hole. Such measurements have even lead us to the astonishing discovery of a whole new category of black holes so-called supermassive black holes living in the hearts of galaxies. More on that to come!